| Abstract
| - We report the results of a 1.2-mm continuum emission survey toward 131 star-forming complexes suspected of undergoing massive star formation. These regions have previously been identified as harbouring a methanol maser and/or a radio continuum source [ultracompact (UC) H ii region], the presence of which is in most instances indicative of massive star formation. The 1.2-mm emission was mapped using the SIMBA instrument on the 15-m Swedish ESO Submillimetre Telescope (SEST). Emission is detected toward all of the methanol maser and UC H ii regions targeted, as well as towards 20 others lying within the fields mapped, implying that these objects are associated with cold, deeply embedded objects. Interestingly, there are also 20 methanol maser sites and nine UC H ii regions within the fields mapped which are devoid of millimetre continuum emission. In addition to the maser and UC H ii regions detected, we have also identified 253 other sources within the SIMBA maps. All of these (253) are new sources, detected solely from their millimetre continuum emission. These ‘mm-only’ cores are devoid of the traditional indicators of massive star formation, (i.e. methanol/OH maser, UC H ii regions or IRAS point sources). At least 45 per cent of these mm-only cores are also without mid-infrared Mid-course Space Experiment (MSX) emission. The ‘mm-only’ core may be an entirely new class of source that represents an earlier stage in the evolution of massive stars, prior to the onset of methanol maser emission. Or, they may harbour protoclusters which do not contain any high-mass stars (i.e. below the H ii region limit). In total, 404 sources are detected, representing four classes of sources which are distinguished by the presence of the different combination of associated tracer/s. Their masses, estimated assuming a dust temperature of 20 K and adopting kinematic distances, range from 0.5 × 101 to 3.7 × 104 M⊙, with an average mass for the sample of 1.5 × 103 M⊙. The H2 number density (nH2) of the source sample ranges from 1.4 × 103 to 1.9 × 106 cm−3, with an average of 8.7 × 104 cm−3. The average radius of the sample is 0.5 pc. The visual extinction ranges from 10 to 500 mag with an average of 80 mag, which implies a high degree of embedding. The surface density (Σ) varies from 0.2 to 18.0 kg m−2 with an average of 2.8 kg m−2. Analysis of the millimetre-only sources shows that they are less massive and smaller than sources with methanol maser and/or radio continuum emission, which collectively have a mean mass of 2.5 × 103 M⊙ and a mean radius of 0.7 pc.
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